Mating between sexually reactive paramecia is characterized by an initial clumping reaction between complementary pairs after random ciliary contact. This highly specific mating reaction is the normal initiation step leading to cell fusion and nuclear exchange. The recognition process appears to be the result of membrane associated proteins which are called mating-type substances or M-agglutins. The project reported here is designed to describe the induction of mating reactivity and to isolate and characterize chemically the mating-type substance.